Ink jet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital data signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. In another process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an image-wise manner onto the surface of the image-recording element, while un-imaged droplets are caught and returned to an ink sump. Ink jet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
The inks used in the various inkjet printers can be classified as either dye-based or pigment-based. A dye is a colorant that is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles. These small particles can be stabilized against flocculation and settling by the use of distinct dispersing agents such as surfactants, oligomers or polymers, or they can be directly functionalized to provide a self-dispersing characteristic. In either case the carrier medium can be a liquid or a solid at room temperature. Commonly used carrier media include water, mixtures of water and organic co-solvents and high boiling organic solvents, such as hydrocarbons, esters, ketones, alcohols and ethers.
Pigment-based inkjet inks are often preferred over dye-based inkjet inks because of the superior image stability typically observed with the pigment-based inks. Self-dispersed pigments in turn are often preferred over surfactant-dispersed, oligomer-dispersed or polymer-dispersed pigments because of their greater stability to a variety of ink formulations and environmental keeping conditions.
Key attributes for ink jet printing on plain papers with inkjet technology include high print density, sharp text quality and high print durability such as high resistance to water, rub and highlighter smear for the printed document on plain paper. Equally important is printing quality uniformity across a large variety of plain papers. It is well known that when printing on plain paper via ink jet vehicle, performance, such as print density, is very sensitive to paper type. Paper type in turn is affected by the paper formulation (e.g. size agent type and amount, filler type and amount, etc.), the manufacturing process, and paper pulp variation. It is highly desirable to have ink formulations that can provide excellent and uniform print quality with low paper to paper variability. In addition, jetting performance of the ink is equally important. This includes, for example, forming stable drops, robust jetting at the desired firing frequency, and maintaining the jetting performance, e.g. constant drop velocity during the extended print head life cycle. Needless to say, inks containing self-dispersed pigments have to date failed to provide all the desired attributes of an ink-jet ink intended for use on plain papers.
Various improvements have been proposed by workers in the field. U.S. Pat. No. 6,638,350B2 describes an ink containing a mixture of self-dispersed pigment and a dye. However, due to the presence of dye in the ink, images printed will suffer image durability, especially with regard to waterfastness. U.S. Pat. No. 6,387,168B1, U.S. Pat. No. 6,533,407B2 and U.S. Applications 2004/0016365A1 describe a pigment based ink containing a mixture of a self-dispersible pigment and a pigment stabilized by a polymeric dispersant. However, the presence of the polymeric dispersant makes the ink viscous and slow to dry. The ink also lacks long term robust firing, especially through a thermal print head.
Therefore, there is clearly a need to provide a pigmented ink composition which can provide laser like print density and text sharpness on plain paper. Said composition also requires good image durability, such as good waterfastness. Additionally, there is a need for ink compositions which, when employed in ink jet printing processes, exhibit stable long-term drop velocity and steady transit time.